The Southern Nevada Health District has confirmed the season’s first cases of West Nile virus, underscoring an urgent public health alert.

According to a press release from the agency, a man in his 60s contracted the non-neuroinvasive form of the virus, while a man in his 70s faced the more severe neuroinvasive type, which causes inflammation of the brain (encephalitis), or inflammation of the membranes surrounding the brain and spinal cord (meningitis).

Both have since recovered. 

The cases come amid record-breaking mosquito activity in Clark County, where 230 mosquito pools from 30 ZIP codes tested positive for West Nile virus as of June 21. Additionally, nine pools from five ZIP codes tested positive for the St. Louis encephalitis virus, according to the health district.

“We have an unprecedented amount of West Nile in Las Vegas at the moment; it’s never been this bad in Southern Nevada this early,” says Louisa Messenger, an assistant professor with UNLV’s School of Public Health. “Part of that, we think, is due to climate change because we have seen such strange weather patterns in the last couple of years … mosquitoes and other insects are so intrinsically linked to temperature and humidity.”

From left, student Karen Figueroa, student Austin Tang, lab coordinator Zoee Sanchez and assistant professor Louisa Messenger pose for a photo at the UNLV Parasitology & Vector Biology morphology lab.

Messenger adds that mosquito activity is soaring, driven by the spread of Aedes aegypti mosquitoes. Known for their daytime biting, these species prefer humans and are testing positive for West Nile virus, according to the health district.

“Aedes Aegypti can test positive for West Nile virus likely because they fed on a West Nile virus-positive human. However, Aedes Aegypti is not considered a competent vector, i.e. the virus may not reproduce and proliferate to be passed to a new human the next time the mosquito takes a bloodmeal,” Messenger says.

“[Aedes Aegypti] is the major vector of dengue virus, which we have not detected yet in Southern Nevada, but was identified in Maricopa County in 2022.”

Aedes aegypti first tested positive for West Nile virus in Southern Nevada in 2017, according to the health district, and they do present a risk to public health. However, the majority of the mosquitoes testing positive for West Nile and St. Louis encephalitis are the Culex species. 

Culex mosquitoes primarily feed on birds but also bite humans, potentially spreading West Nile virus and St. Louis encephalitis.

“I think a lot of the rhetoric I hear from people is we’ve never had mosquitoes in Vegas. We have had them, just not at these densities,” says Messenger.

She and her team of researchers have been working closely with the health district to monitor mosquito species across the valley. Her laboratory is performing genetic analyses of the virus samples and mosquitoes to support the health district’s surveillance activities.

“Mosquitoes reproduce every 10 to 14 days so they’re really rapidly growing arthropods … they can become resistant to chemicals quickly … which makes it problematic if you try to do any type of control,” she says. 

The health department and scientists are now emphasizing the importance of individual and community participation in preventing mosquito-borne illnesses. To start, personal protection is key. Wearing EPA-approved insect repellent, such as topical lotions or sprays, can significantly reduce your chances of being bit. For those spending time in areas with high mosquito activity such as hiking trails, wearing insecticide-treated clothing is recommended.

“You can go to REI and get the clothes that have pyrethroid insecticides in it—that’s the best thing you can do to prevent biting,” Messenger says. Long sleeves and pants offer additional protection against the mosquitoes active in the day and night.

However, individual efforts alone aren’t enough. Community-level actions are crucial, Messenger adds. Public health officials recommend eliminating any standing water at home, which is the preferred breeding ground for mosquitoes. This includes untreated pools, stagnant water in plant pots, children’s toys and any other containers that may collect rainwater. It’s surprising how small amounts of water, even less than a milliliter in items hidden under bushes, can become mosquito breeding sites.

Mosquito activity can be reported to the health district’s mosquito surveillance program, which can be reached at 702-759-1633. To report a green pool, people should contact their local code enforcement agency, the health district says.

It’s also important to recognize the symptoms of West Nile virus and St. Louis encephalitis, if bitten. Most people who are infected won’t experience symptoms, but 20% may develop mild signs such as fever, headache, body aches, joint pains, vomiting, diarrhea or a rash, according to the Centers for Disease Control and Prevention. These symptoms typically appear between 2 to 14 days after the bite and can last for several weeks.

While these symptoms may very well point to another kind of typical infection, it doesn’t hurt to let your physician know if you’re been in recent contact with mosquitoes so that they can test for the diseases.

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